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Typically recommended is twice the size of your OS X partition/drive.
Really depends on how much data is kept on and moved on/off the partition.

ex 1
My wife could have a 1TB drive in her computer - meanwhile the total data on her current machine that she has been using for 6 yrs now is <40GB. Her current Time machine backup with 3 yrs+ of backups is <50GB. So, the double the partition size is irrelevant related to her usage.

ex 2
Meanwhile, the 250GB SSD in my MacBook Air with only 120GB used, my 1 year old TM backup is just under 600GB in size. As you can see, the double size would not work for me either or my oldest backups would already have been deleted in less than a year.

Both machines run with the typical hourly TM backups.

Only you know how much data you typically use and how much additional stuff you move on and off the drive and whether any of that really needs to be backed up.

I cannot be held responsible for the things that come out of my mouth.
In the Windows world, most everything folks don't understand is called a virus.